It is known that a resin sheet comprising polypropylene, high-density polyethylene, and an inorganic filler is excellent in vacuum moldability as described in U.S. Pat. No. 4,567,089 corresponding to Japanese Laid-Open patent application No. 165654/84. The vacuum molded articles obtained therefrom are useful as interior materials of a refrigerator or packaging trays. The above U.S. Patent discloses that the molded articles obtained have a draw ratio of 1.0 and a uniform thickness.
Vacuum molding, pressure air molding, and vacuum molding with a pressure air (inclusively referred to as differential pressure molding or so-called "thermoforming") and press molding are advantageous in terms of cost incurred for molds and devices as compared with injection molding (cf. Japanese Laid-Open patent application No. 152736/80). This advantage leads to a reduced cost of products and a cheap supply of products.
However, drawing limits of the conventional deep drawing sheets for obtaining containers or trays having a practically uniform thickness were unsatisfactory. For example, the possible highest draw ratio of a noncrystalline resin sheet (e.g., a polyvinyl chloride sheet and a high-impact polystyrene sheet), a polypropylene sheet or a high-density polyethylene sheet is 1.5, 0.8 or 1.0, and the draw ratio of the aforesaid resin sheet of U.S. Pat. No. 4,567,089 which comprises polypropylene, polyethylene and an inorganic filler is 1.5 at the highest as shown in Comparative Example 2 hereinafter described.
The term "draw ratio" as used herein means a ratio of depth D to width W of a molded container as shown in the accompanying drawing (hereinafter referred to as D/W ratio).
If molded articles, such as containers and trays, having a D/W ratio of at least 2 could be obtained by differential pressure molding or press molding, they would develop a broader market.